1. Field of the Invention
The present invention relates to a wireless communication system, especially to a Channel Quality Indicator (CQI) report method for frequency scheduling in the wireless communication system.
2. Description of the Related Art
Now, the 3rd Generation Mobile Communication System Partnership Project (referred to as 3GPP) standardization organization has commenced on Long-term Evolution (referred to as LTE) to existing system criteria. Among numerous physical layer transmission techniques, OFDM (Orthogonal Frequency Division Multiplex) becomes a challenging one in all downlink solutions, for it bears advantages such as higher spectrum utilization efficiency and lower processing complexity.
In nature, the OFDM is a multi-carrier modulation communication technique, and its basic principle is to divide a high rate data stream into multiple low rate data streams to transmit via a group of orthogonal sub-carriers simultaneously. Because of the features of multi-carrier, the OFDM technique bears superior performance in many aspects. (1) A remarkable superiority of the OFDM technique lies in that: inter-symbol interference (ISI) can be completely eliminated in the case that the channel delay is less than the length of CP (cyclic prefix, which is introduced in by adding guard spacing to each symbol) since data is transmitted through multiple sub-carriers in parallel and the length of symbol in each sub-carrier is correspondingly increased but with no sensitivity to channel delay. In this way, each sub-carrier experiences a flat fading channel. (2) The OFDM technique bears high spectrum utilization efficiency. In frequency domain, OFDM signals overlap actually. This overlap improves spectrum utilization efficiency in great extent. (3) The OFDM technique bears strong ability in anti-narrowband interference or resisting frequency selective fading. Through channel coding and interleaving, the frequency diversity effect and the time diversity effect can be achieved in OFDM so that either narrowband interference or frequency selective fading can be effectively resisted. (4) In the technique of OFDM, the modulation can be realized through the base-band IFFT, and IFFT/FFT bears available fast calculation method and can be conveniently implemented in a DSP chip and hardware structure.
In order to obtain larger throughput of data transmission in the OFDM wireless transmission system, frequency allocating can be applied in the data transmission. Each transmission channel of user usually suffers from different frequency domain fading in different frequency bands. Therefore, each user bears different channel quality in a frequency band. With frequency scheduling, frequency bands can be allocated to the users with better channel conditions to maximize the throughput of data transmission.
To implement frequency scheduling, it is necessary for the base station to learn about channel quality of the UE in each frequency band to be allocated. Therefore, it is necessary for each UE to report the CQI of all frequency bands to the base station. The number of information bits necessary for a single user to report the CQI to the base station equals the number of total information bits necessary for the corresponding CQI report in all frequency bands. Therefore, for each UE, the more allocated frequency bands to report, the heavier the corresponding necessary uplink signaling overhead is.
During a frequency allocating process, more information and more detail the base station obtains for channel quality of each UE, better scheduling gain will be reached according to the information. However, the more information report is, the heavier the overhead of uplink signaling is, and more impact is caused to uplink data transmission. Thus, it is necessary to turn to some better methods to reduce the uplink signaling overhead in CQI reporting as much as possible on the premise that specific frequency allocation gain is guaranteed.
At present, there are many methods for reducing corresponding signaling overhead in CQI reporting. A most often used method is that the UE reports the CQIs of some frequency bands with better channel qualities to the base station. But in this method, besides the CQI information bits of the corresponding report frequency bands extra information bits are necessary to be transmitted to the base station, indicating which frequency bands the reported CQIs belong to. Since the number of frequency bands to be reported is part of the number of entire frequency bands, corresponding signaling overhead is greatly reduced in this method.
At present, when reporting the CQIs of partial frequency bands, a bit-mapping method is used to indicate which frequency bands the reported CQIs belong to. With this method, if the system has total M CQIs to be reported, a sequence of M bits is adopted for indication, setting the sequence number of the bit corresponding to the frequency band needed to report as “1”, and the sequence number of the bit corresponding to the frequency band needed not to report as “0”.
As shown in FIG. 1, with the bit-mapping method in indicating the frequency bands to which the reported CQIs belong, the CQI report signaling contains a bit mapping sequence, each bit of which corresponds to a frequency band. If the bit is set as “1”, the corresponding frequency band is the one to be reported by CQI; and if the bit is set as “0”, the corresponding frequency band is not the one to be reported by CQI.
From the descriptions above, the signaling bits necessary for CQI reporting of partial frequency bands include two parts. One is to indicate the frequency band (which needs to be reported) with the bit-mapping method. The other is to report CQIs of the frequency bands.
With the bit-mapping method to indicate the frequency bands which need to be reported, the number of the necessary information bits depends on the number of total CQI report frequency bands but have nothing with the number of frequency bands which need to be reported. The number of information bits necessary for frequency band indicating equals the total number of frequency bands in the system. Therefore, if there are comparatively more frequency bands in the system, the signaling overhead is still heavy. It is necessary to adopt some new methods to perform further optimization on CQI reporting. For the CQI reporting of the selected frequency bands, some optimization methods may also be adopted to reduce the signaling overhead for total CQI reporting.